Numerical Simulation Of Flow And Heat Transfer Characteristics Of R1234yf And Its Mixture Refrigerant In Micro-channels

R1234yf refrigerant is widely used in automotive air conditioning because of its low GWP and excellent environmental performance.In this paper,the boiling heat transfer and flow characteristics of R1234yf in microchannel are studied by numerical simulation.At the same time,R1234yf has weak flammability,in order to meet the requirements of environmental protection,safety and efficiency,the boiling heat transfer and flow characteristics of R1234yf/R134a with mass ratio of 0.89/0.11 in microchannel are studied.Finally,the heat transfer and flow characteristics of R134a and R1234yf are compared.Based on the mixture multiphase flow model and UDF phase change module,the boiling heat transfer of R1234yf、R134a and R1234yf/R134a mixture refrigerant in a rectangular microchannel with a hydraulic diameter of 1 mm and a length of 200 mm is simulated.The effects of heat flow,mass flow rate,vapor quality and channel aspect ratio on the flow and heat transfer characteristics are studied.The simulated operating range is:saturation temperature 278.15K,heat flow(3 kW/m2～12kW/m2),mass flow rate(150 kg/(m2·s)～300 kg/(m2·s)),vapor quality(0～0.2),aspect ratio(0.6～1.4).The results show that the vapor volume fraction and section temperature of R1234yf,R134a and R1234yf/R134a all increase along the length of the tube,and the fluid velocity increase with the increase of the vapor component,the pressure along the tube decreases due to the friction between the gas and liquid phases、the friction between the fluid and the wall.The heat transfer coefficient and pressure drop of the three refrigerants increase with the increase of mass flow rate,heat flow and vapor quality.When x=0.1,the heat flux density increases from 3 kW/m2 to 12 kW/m2,the heat transfer coefficient of R1234yf,R134a,R1234yf/R134a increases by 21.4%,19.6%,20.03%respectively,and the pressure drop increases by 19.8%,14.5%,18.9%respectively;when the mass flow rate increases from 150 kg/(m2·s)to 300 kg/(m2·s),the heat transfer coefficient of the three refrigerants increases by 120%,117%,125%respectively,and the pressure drop increases by 239%,229%,237%.With the increase of aspect ratio,the heat transfer coefficient of the three refrigerants decrease,and then increase.When the height and width of the channel are all 1 mm,the heat transfer coefficient is the smallest,and the pressure drop decreases gradually with the increase of aspect ratio,At the same time,when the aspect ratio is 1.4(H=1.2 mm,W=0.86 mm),the comprehensive evaluation factors of the three refrigerants are the largest,and the comprehensive heat transfer effect is the best.Under the same operating conditions,R134a has the highest boiling heat transfer coefficient and pressure drop,R1234yf/R134a is the second,and R1234yf is smallest.Taking the comprehensive evaluation factor R=(j/j0)/(f/f0)1/3 to compare the comprehensive heat transfer effect of the three refrigerants,the results show that R134a has the best comprehensive heat transfer effect under the same working condition,R1234yf/R134a is the second,R1234yf has the worst comprehensive heat transfer effect.When x=0.1,q=9 kW/m2,G=210 kg/(m2·s),aspect ratio is 0.6,the comprehensive factor of R1234yf/R134a is 2.88%lower than R134a,and 1.27%higher than R1234yf.However,because the mixture refrigerant is more environmentally friendly(GWP<150)and nonflammable,R1234yf/R134a mixure refrigerant with a mass ratio of 0.89/0.11 can be considered as a substitute for R134a.